The prior art is replete with numerous examples of various distributor designs employed in various refrigeration arrangements.
As a general matter, refrigeration evaporators have multiple parallel circuits which require some type of a device to evenly distribute equal amounts of refrigerant to each of the circuits. This “equal distribution” feature becomes critically important with evaporators that are fed by means of dry or so-called “direct” expansion. In this regard, and in a dry expansion system, it has been understood that the flow of refrigerant to the evaporator is controlled by an expansion valve operating either on a thermal-mechanical, that is thermostatic basis, or an electronic control principal. This expansion valve regulates the flow of refrigerant in response to the cooling load that is imposed on the evaporator.
With respect to earlier prior refrigerant evaporators, and especially direct expansion types, the refrigerant which is supplied experiences a pressure drop typically across the expansion valve, which in turn, normally produces some adiabatic boiling of the refrigerant. This adiabatic boiling results in a “flash gas” and a two phase fluid flow, that is, gaseous or vapor like refrigerant mixed with liquid refrigerant especially at the entrance to the evaporator circuits. The prior art distributor's function was to divide this mixture of vapor and liquid coming in from the expansion valve equally to the multiple parallel evaporator circuits. As the refrigerant passes through the evaporator circuits, it is boiled, that is evaporated, and then finally superheated. An equal amount of refrigerant distributed to the entrance of each of the circuits typically insures an equal amount of superheat at the exit of each of the same evaporator circuits. It has been well understood that uniform superheating of the refrigerant vapor at the exit of each circuit is needed for stable modulation of the expansion valve. The prior art has also taught that the equal distribution function of the distributor is also important to the proper operation of near-dry expansion evaporators. As with dry expansion systems, near-dry expansion also introduces a two phase mixture of liquid and vapor refrigerant at the entrance of the evaporator. However, unlike dry expansion, the refrigerant does not evaporate completely such that the condition of the refrigerant at the exit of the circuit is saturated or slightly “wet,” that is, with only a small amount of liquid remaining. The prior art distributor designs have heretofore used pressure drop across an orifice plate and through small diameter distributor tubes, which are typically called “leads” to thoroughly mix the vapor and liquid refrigerant just prior to entering the evaporator circuits. Typically, orifice plates are selected for pressure drops of approximately 25 lbs per square inch, and distributor leads for a pressure drop of about 10 to 15 lbs per square inch. This has resulted in a total pressure drop across a distributor assembly of sometimes between about 35 to 40 lbs per square inch at the design refrigerant flow rate condition on which it is employed.
Those skilled in the art will recognize that ammonia is produced in large quantities for use in agriculture, power generation and other industries. It has also long been known that ammonia makes an excellent refrigerant with outstanding thermodynamic and heat transfer properties. Moreover, ammonia is naturally occurring and also has an Ozone Depletion Potential (ODP), and Global Warning Potential (GWP), of zero. In addition to the foregoing, ammonia has traditionally been used in industrial refrigeration, but it is finding wider acceptance in other applications such as air conditioning and the like. In this regard, it has long been known that ammonia is toxic and flammable. Therefore, it would be desirable to develop a refrigeration system employing ammonia and which would use a minimal charge inventory circulating in the system in order to avoid hazards should the refrigeration system be breached. Those skilled in the art will readily recognize that a smaller refrigerant charge in the refrigerant system translates to less risk in the event of a leak or a release of the refrigerant to the immediate ambient environment.
Because of the risks noted above, dry or near-dry expansion operations result in the smallest possible refrigerant charge in the evaporator itself, and also minimizes the refrigerant charge in various other parts of the refrigeration system, that being, the liquid lines, liquid receivers, and other components. In view of the wide interest in reducing refrigerant charges in ammonia systems solely for safety reasons, designers and operators of ammonia refrigeration systems have long been motivated to use dry expansion with ammonia as a refrigerant. One of the principal properties of ammonia which makes it desirable as a refrigerant is its high latent heat of vaporization. This physical property results in relatively low mass flow rates for a given cooling capacity. Lower mass flow rates means smaller liquid pipes, and pump sizes, and low pumping power. However, the low mass flow rate of ammonia also results in very small distributor orifice and lead sizes. The very small orifices and small lead sizes result in several serious operational problems which have yet to find acceptable solutions. These problems include, among others, the deposit of scale and dirt from the interior of pipes, valves, and vessels in various locations in a system. For example, this scale and dirt can partially or completely plug orifices and/or leads thereby blocking the flow of the refrigerant. In addition these small orifice sizes can result in the overall refrigeration design having a cooling range of operation that is relatively narrow, that is, the evaporator cannot be operated efficiently under cooling loads which are significantly higher or lower than the design condition of the evaporator. It has long been known that a typical effective operating range of only about 50% to 150% of the rated capacity of the distributor is usually available. In addition to the foregoing, and during hot gas defrosting of an evaporator, the flow of gas through the distributor is severely limited. The high pressure drop of the refrigerant hot gas can cause a number of problems including longer than desired defrost times and, vibration damage may occur in the form of cracks which form in the distributor leads.
In addition to the several problems noted above, compressor lubrication oil which is often used in ammonia refrigeration systems sometimes becomes mixed with the refrigerant. This lubrication oil is typically immiscible and becomes very viscous and “tar-like” at low temperatures. If these immiscible oils reach the expansion valve they can then be cooled to the evaporator temperature. At this temperature, they can foul the distributor orifice and/or distributor tubes resulting in improper operation of the distributor and reduced evaporator capacity.
Currently, conventional distributor designs provide no convenient means of separating and capturing these immiscible oils before they reach the distributor. In addition to all the shortcomings noted above, conventional distributor designs also require that the expansion valve be mounted in close proximity or directly on to the distributor. Consequently, this installation location causes the expansion valve to be typically located within the refrigerated space. In view of the risks associated with a leak of ammonia refrigerant, especially in a refrigerated space, earlier prior art designs have not been widely utilized because a refrigerant leak would tend to interrupt operations, cause product damage, and could cause injury to workers.
It has long been known that it would be desirable to provide an improved refrigerant distributor which may be utilized with an ammonia evaporator heat exchanger, and which avoids the detriments individually associated with the prior art devices and practices employed heretofore.